To me, 5 stars means there were no problems and I would use the service again. I only give less stars if there are problems. I personally would never consider buying a product or using a service if it had 3stars reviews.
I work as a part time private tutor and I would be very sad and disappointed if one of my students gave me 3 stars without explaining why or giving me a chance to improve. I would definitely call them and ask for explanation.
To me, three stars mean "I won't complain but I will never use this service again".